How can ethanol enhance direct interspecies electron transfer in anaerobic digestion?

Anaerobic digestion (AD) of organic waste to produce biogas is a mature biotechnology commercialised for decades. However, the relatively recent discovery of direct interspecies electron transfer (DIET) brings a new opportunity to improve the efficiency of biogas technology. DIET may replace mediated interspecies electron transfer (MIET) by efficient electron transfer between exoelectrogens and electrotrophic methanogens, thereby enhancing yields and rates of biogas production. Ethanol, as the initial electron donor in the discovery of the DIET pathway, is now a "hot topic" in the literature. Recent studies have indicated that ethanol in AD functions not only as the substrate, but also as the precursor to stimulate DIET by enriching exoelectrogens and electrotrophic methanogens for co-digesting complex organic wastes. This review aims to highlight the state of the art and recent advances in ethanol-based DIET in AD. The DIET associated reactions of ethanol oxidation and carbon dioxide reduction are assessed by thermodynamic analysis to reveal the extent of the potential for improvement of the AD processes that utilizes DIET pathways. Three ethanol-based DIET strategies are discussed: (1) ethanol as the sole substrate supplemented with conductive materials in AD, (2) ethanol co-digestion with complex substrates and (3) ethanol-type fermentation prior to AD. This review aims to chart the pathways for improved AD performance by utilizing ethanol-based DIET in specific treatments of biological wastes.